Is this situation a result of an Apple security initiative or did I mess up some setting? And if so, what's the fix?
I don't think it is an Apple initiative because I am not seeing it in Mail 6.8 (OS X 10.8.5) nor am I seeing it in iOS 7 Mail. Rather, I think if you will view the initial message and banner in raw source you will find JavaScript (not Java) that drives the loading of the full message text. Some senders of large messages are using this technique because so much email is being opened on smart phones with small screens, limited memory, and pernicious data plans. Basically it is a variation on "…if you are having trouble viewing this message click here". This would also be a good way to get around spam filters by substituting large image files for scannable text and it could even be used as a vehicle to distribute malware.
The fix is to ask to be removed from the mailing list or flagging the messages as Junk mail. If you have the Developer's menu installed in Mail disable Javascript (I'm not sure what would happen if you do that, but it would be interesting to find out).
